_interesse.html.haml

= simple_form_for('/enviarmail', html: { class: 'form-horizontal' }) do |f|
  = f.error_notification
  .container-fluid
    .form-inputs
      .form-group
        = label_tag "Nome", nil, class: "control-label"
        = text_field_tag :name, nil, class: "string required form-control"
      .form-group
        = label_tag "Email", nil, class: "control-label"
        = text_field_tag :email, nil, class: "string required form-control"
  .form-actions.pull-right
    = button_tag "Enviar", class: "btn btn-warning"
  %br
  %br

discos_controller.rb

class DiscosController < ApplicationController
  before_action :set_disco, only: [:show, :edit, :update, :destroy, :interesse, :enviarmail]

  # GET /discos
  # GET /discos.json
  def index
    @discos = Disco.all
  end

  # GET /discos/1
  # GET /discos/1.json
  def show
  end

  def interesse
    # Aqui era pra ter alguma coisa, como os dados recebidos pelo formulário
  end

  def enviarmail
    EnviarEmail.send_mail(@disco).deliver
    respond_to do |format|
      format.html {render :show, notice: "Email enviado com sucesso!"}
      format.js
    end
  end

interesse.js.haml

$(".modal-body").empty();
$('.modal-body').html('#{escape_javascript(render 'interesse')}');
$("#modal-interesse").modal('show');

routes.rb

  resources :discos do
    member do
      get 'interesse' => 'discos#interesse'
      get 'enviarmail' => 'discos#enviarmail'
    end
  end

show.html.haml

  %p
      %strong Nome:
      = @disco.name
  %p
    %strong Imagem:
    = image_tag @disco.image.url(:medium)
      
  = link_to interesse_disco_path, {class: 'btn btn-warning', 'data-toggle' => 'modal', 'data-target' => '#modal-show', :remote => true} do
    %i.fa.fa-heart-o 
    Estou interessado!

#modal-interesse.modal.fade{:role => "dialog", :tabindex => "-1"}
  .modal-dialog
    .modal-content
      .modal-header
        %button.close{"aria-label" => "Close", "data-dismiss" => "modal", :type => "button"}
          %span{"aria-hidden" => "true"} ×
        %h4.modal-title Tenho interesse no disco: #{@disco.name}
      .modal-body
      .modal-footer
        %button.btn.btn-default{"data-dismiss" => "modal", :type => "button"} Close